Field of the Invention
This invention relates generally to a sewing machine and, more particularly, to a sewing machine suitable for use in quilting beddings or the like.
Some of industrial sewing machines for use in sewing-up of bed quilts or the like are provided with a sewing machine head having a needle, a bobbin unit and a working table disposed at a position downward of the machine head for supporting a workpiece such as a bed quilt or the like so as to be interposed between the machine head and the bobbin unit. Such a kind of sewing machine is constituted so as to stitch various kinds of decorative patterns by horizontally moving the working table in its X and Y directions. Some of such sewing machines present an excellent sewing efficiency when the direction of the machine head and bobbin unit thereof are controlled so as to always coincide with the sewing direction in order to beautifully sew a complicated pattern.
One example of such machine head and bobbin unit will be described with reference to FIG. 1. In FIG. 1, a machine head 1a is rotatably mounted on a horizontally extending hollow arm portion 2a of a machine body 2, the arm portion 2a being of a square cross-section. A pulley 3a is fixedly mounted on a hollow shaft 4a which is fixedly secured to the machine head 1a and rotatably mounted on the arm portion 2a through a bearing 5a and a collar 6a. A drive force of a head drive motor (not shown in FIG. 1) is transmitted to the pulley 3a through a belt 7a to rotate the machine head 1a about an axis of the hollow shaft 4. A shaft 8a of less diameter than the shaft 4a is disposed coaxially of and rotatably mounted on the hollow shaft 4a. Fixedly mounted on the upper end of the inner shaft 8a is a pulley 9a which is rotated by a needle drive motor (not shown in FIG. 1) through a belt 10a. The rotation of the inner shaft 8a is transmitted through a pulley mounted on the lower end thereof (not shown) and a belt 11 to a pulley 12 which is fixedly mounted on a shaft 13 extending perpendicularly to the shaft 8a. The rotation of this horizontally disposed shaft 13 is transmitted to an input shaft of a well-known crank mechanism which vertically reciprocally moves a needle 14 slidably mounted on the machine head 1a.
A bobbin drive unit 1b is provided below the machine head 1a. The bobbin drive unit 1b is rotatably mounted on another horizontally extending arm portion 2b of the body 2. The bobbin drive unit 1b is rotated about an axis of an outer shaft 4b by the head drive motor through a belt 7b and a pulley 3b in synchronism with the machine head 1a. The internal mechanism of the bobbin drive unit 1b is driven by the needle drive motor through a belt 10b, a pulley 9b and an inner shaft 8b. A workpiece such as a bedding to be quilted is interposed between the machine head 1a and the bobbin drive unit 1b with the peripheral portion thereof secured to a working table in the form of a frame (not shown in FIG. 1).
The sewing machine with the aforesaid arrangement has such a disadvantage that when the direction of the machine head 1a is changed by actuating the head drive motor with the needle drive motor stopped, the needle 14 vertically moves due to the relative rotational movement of the inner shaft 8a to the outer shaft 4a. In other words, the needle 14 vertically reciprocally moves in accordance with the difference between the rotations of the inner shaft 8a and the outer shaft 4a.
On the other hand, when the sewing operation is changed from a sewing operation for a certain decorative pattern to that for another decorative pattern which is not continuous with the former pattern (i. e., in the case of what is called "skip sewing") or in the case of a tracing operation wherein a sewing program is tested by moving the machine head 1a and the working table in the same manner as the actual sewing operation without driving the needle drive motor, the needle 14 must be held in its elevated position irrespective of the rotational movement of the head 1a to prevent the workpiece from being damaged.
To prevent this movement of the needle 14, the conventional sewing machine has been equipped with a clutch mechanism to transmit the drive force of the head drive motor to the inner shaft 8a so that the inner shaft 8a is rotated in synchronism with the machine head 1a. In this manner, the relative rotational movement of the inner shaft 8a to the machine head 1a has been prevented.
However, the use of the clutch mechanism lowers the reliability of the sewing machine due to possible slip of the clutch which may occur as a result of the aged deterioration of the clutch. Also, the use of the clutch mechanism causes a risk that a workpiece can be damaged when the work table is moved under such condition that the clutch is connected when the needle is not sufficiently raised.